The requirements for dental alloys include adequate working time in which to fill the tooth cavity as well as to carve the tooth's surfaces to conform to dental anatomy, good handling properties, minimum dimensional changes with lapse of time, sufficient mechanical and physical properties to permit normal biting and chewing over a long period of time, and finally superior tarnish and corrosion resistance.
The American Dental Association has set qualitative and quantitative elemental restrictions for amalgam alloys in its Specification No. 1 (Revised), which include in weight percentages 65% or more silver, 29% or less tin, 6% or less copper, 2% or less zinc and 3% or less mercury. While alloys made under said specification are adequate, they exhibit relatively low mechanical properties, and are corrosion prone.
To improve mechanical properties of amalgam alloys, it has been proposed to admix an adequate supply of copper-silver eutetic particles with the alloy made in accordance with said Specification. When such a high-copper alloy is mixed with mercury, and the resultant amalgam microscopically analyzed, there are many non-reacted silver-tin particles dispersed in the matrix which advantageously affect the mechanical properties of the amalgam. U.S. Pat. No. 3,305,356 discloses an example of such a high-copper alloy for amalgam.
Although an amalgam with such a high copper content has improved mechanical properties, it has lower tarnish resistance to oral sulfides. Although the physical and mechanical properties of the amalgam are not affected by tarnish, the latter can seriously affect aesthetics and even possibly health.
In order to improve upon both the mechanical and chemical properties and tarnish resistance of dentral amalgams, we previously developed a group of alloys containing silver, copper, tin and indium (c.f., Japanese Patent No. SHO 50- 147412), which was superior to the prior art alloys in mechanical properties and tarnish resistance. However, we have now found a composition range of such alloys surprisingly superior to the alloys of the Japanese Patent.
The prior art includes other amalgamating alloys that contain indium. For example U.S. Pat. No. 1,959,668 to Gray discloses dental alloys which comprise silver, tin, and relatively small amounts of tin and copper and up to 25% by weight indium. Additional prior art which discloses dental alloys which contain indium includes British Dental Journal: The Effects of Adding Indium to Dental Amalgams, MacCulloch, vol. 123, No. 11 (12/5/67). None of the dental alloys disclosed therein, however, has satisfactory compressive strength or workability. None of these references discloses dental alloys which contain both indium and copper wherein the weight percent of the latter is more than a small amount.
Generally speaking, alloys for dental amalgams are composed of silver, tin and a small amount of copper and, when intermixed with mercury at about 1:1 weight ratio, have an amalgam structure of non-reacted alloy particles dispersed in an amalgam matrix produced by a reaction with mercury. This amalgam mass includes a silver-tin phase produced by reaction of silver and tin with mercury and a tin-mercury phase (GammaII) produced by a reaction of tin with mercury, which is weak mechanically and chemically. However, if tin is not added to a certain extent, the powdered alloy reacts insufficiently with mercury and, moreover, hardens quickly, so that not only is it inconvenient for use but also the silver-tin phase itself may become insufficient in strength.
However, it has been found that, if the powdered alloy contains a large amount of copper, tin reacts with copper to form Cu.sub.6 Sn.sub.5, a stable intermetallic compound. Thus, the structure becomes such that relatively large particles of non-reacted alloy are surrounded by relatively small crystal grains of Cu.sub.6 Sn.sub.5 with the remaining space filled with the amalgam matrix. This amalgam matrix is composed of a silver-mercury phase and includes substantially no tin-mercury phase which is chemically and mechanically weak.
The composition optimum to produce the intermetallic compound Cu.sub.6 Sn.sub.5 consists of 1 part by weight of copper and 2 parts by weight tin. Accordingly, it is believed that most of the tin in the alloy is consumed for producing said intermetallic compound and for reinforcing the silver-tin phase, rather than for producing the mechanically and chemically weak tin-mercury phase. However, alloys containing over 6% copper by percentage weight have poor tarnish resistance.